Fix screen blanking in documentation

@nathan-contino See #2959 and #3065. Then see https://forums.raspberrypi.com/viewtopic.php?p=2139724#p2139847.

Looks like screen blanking works slight differently now, can you fix up `computers/configuration/screensaver.adoc`. Thanks!

PS. Bookworm branch has been merged down to develop, so we're all on develop now. 